It’s a bittersweet birthday for Mac OS X. We reported yesterday that the man behind OS X, Bertrand Serlet, is leaving Apple. But, that doesn’t mean there’s not reason to celebrate this operating system’s great evolution over the past 10 years.

Mac OS X v10.0 launched 10 years ago today, March 24, 2001, and was the first Apple operating system based on the UNIX platform. It was also the first to be named in the “big cat” series that is continuing today. It was created as a next-generation operating system, and was the tenth major version of Apple’s OS — hence the “X”. Although the first release suffered from a number of performance issues, Cheetah still received critical acclaim.

Mac OS X introduced some important developer tools, one of which was its integrated development environment Xcode that allowed developers to create apps for Mac computers whether they run on PowerPC or Intel processors.

Over the 10 years, we’ve seen six major revisions of Mac OS X, from Cheetah (10.0), to Puma (10.1), to Panther (10.3), to Tiger (10.4), to Leopard (10.5), and finally to Snow Leopard (10.6). Apple has just released the first beta for Mac OS X 10.7 called Lion. Let’s take a look at the different versions throughout the last decade:

Cheetah: March 24, 2001
The first version, Cheetah, had an Aqua interface, which moved away from its professional-looking Macintosh style, and matched its software design to the hardware design of its Macs during that time. Cheetah introduced the Mac world to the Dock, Terminal, Mail, Address Book, TextEdit, AppleScript and PDF support, along with full multitasking. As we mentioned above, the first version was pretty clunky, and slow, but that meant there was only room to improve.

Jaguar: August 24, 2002
Finally ready for the mainstream, Jaguar introduced stability and speed fixes. It featured a revamped Finder and Address Book, as well as Quartz Extreme, Bonjour, Inkwell, and network support for Windows, among other things. OS X was on its way up.

Panther: October 24, 2003
Along with revisions to existing features, Panther introduced Expose, Font Book, iChat, FileVault, and Safari. It was also the first version of OS X that wasn’t compatible with some older Macs, like the earliest Mac G3s and PowerBook G3s.

Tiger: April 29, 2005
New features appeared, including Dashboard, Automator, Dictionary, Front Row, and Quartz Composer. Tiger also released a version 10.4.4 in January 2006, which made it available to Intel computers.

Leopard: October 26, 2007
Tiger stayed as Apple’s OS for a almost two-and-a-half years before Leopard was finally released. It featured Back to My Mac, Spaces, Boot Camp, and Time Machine. It was the first post-iPhone version of OS X, and was also the first not to run in Classic Mode.

Snow Leopard: August 28, 2009
Apple seemed to really be stretching the release dates. After another almost two years, Snow Leopard was released. It was the first version not to support PowerPC Macs, and AppleTalk was also no longer supported. Other than the upgrade from Cheetah to Puma, it was the first version to not have the $129 price tag. It was offered for $29 to those who already owned Leopard. Updates to the Finder, Preview, Quicktime X, and Safari were also added.

Lion: Summer 2011
Apple launched a developer preview of Mac OS X Lion through the Mac App Store a month ago. From that, we learned it will include a new version of Mail; AirDrop, which will be a remarkably easy way to copy files wirelessly from one Mac to another; Versions, which automatically saves successive versions of your document as you create it; Resume, which brings your apps back to exactly how you left them when you restart your Mac, or quit and relaunch an app; AutoSave, which automatically saves your documents as you work; the new FileVault, that provides high performance full disk encryption for local and external drives; and the Mac App Store.

If you want to take even more of a trip down memory lane, Ars Technica has posted its review of Mac OS X Public Beta from October 15, 2000.